The present invention relates to a displacement control valve used for a variable displacement compressor that adjusts the pressure in a pressure control chamber by introducing a refrigerant in the discharge pressure region of the compressor into the pressure control chamber through a supply passage and releasing the refrigerant in the pressure control chamber to the suction pressure region of the compressor through a bleed passage, thereby controlling displacement of the compressor.
In a variable displacement compressor having a pressure control chamber that accommodates a swash plate whose inclination angle is variable, the inclination angle of the swash plate decreases as the pressure in the pressure control chamber rises. This decrease of the inclination angle increases the stroke of a piston, thereby to increase the displacement of the compressor. On the other hand, the inclination angle of the swash plate increases as the pressure in the pressure control chamber falls. This increase of the inclination angle decreases the stroke of the piston, thereby to decrease the displacement of the compressor.
Unexamined Japanese Patent Publication No. 2001-153044 discloses a displacement control valve operable to open and close a supply passage for introducing a refrigerant gas from a discharge pressure region into a crank chamber (pressure control chamber). The displacement control valve includes a solenoid and a pressure sensing means operable to sense a pressure difference between two points at the discharge pressure region to operate a valve body. As the flow rate of the refrigerant gas increases, the pressure difference between the two points increases. According to the increase of the pressure difference, the pressure sensing means displaces the valve body in the direction which causes a valve hole to be opened. Thus, the pressure in the crank chamber rises, and the displacement of the compressor is decreased. On the other hand, as the flow rate of the refrigerant gas decreases, the pressure difference between the two points decreases. According to the decrease of the pressure difference, the pressure sensing means displaces the valve body in the direction which causes the valve hole to be closed. Thus, the pressure in the crank chamber falls, and the displacement of the compressor is increased.
The displacement control valve includes the solenoid that generates an electromagnetic forces acting on the valve body against the above pressure difference. The displacement control valve varies the opening degree of the valve hole in accordance with the variation of the value of an electric current (a duty ratio) supplied to the solenoid. The value of the electric current (the duty ratio) supplied to the solenoid is determined by a controller. For example, the controller determines the value of the electric current (the duty ratio) supplied to the solenoid based on the difference between a set target room temperature and a detected room temperature.
When the variable displacement compressor operates with insufficient refrigerant gas, the room temperature does not fall to the target room temperature because of the insufficient flow rate of the refrigerant gas. In accordance with the situation, the controller changes the value of the electric current (the duty ratio) supplied to the solenoid to the maximum so that the inclination angle of the swash plate is changed to the maximum. Even in a state where the rotation speed of a rotary shaft becomes high to increase the flow rate of the refrigerant gas, the variable displacement compressor operates at its maximum displacement. Such high rotation speed of the rotary shaft and large displacement operation produce a great load acting on the compressor, more particularly on the swash plate, which is undesirable in view of reliable operation. Additionally, the discharge pressure does not increase because of the insufficient refrigerant gas. In a variable displacement compressor including a hinge mechanism as disclosed in Unexamined Japanese Patent Publication No. 2004-108245, which allows the swash plate to freely move in the axial direction of the rotary shaft, the inertial force of the piston exceeds compressive reactive force, so that the inclination angle of the swash pate at the maximum displacement operation may exceed a predetermined maximum inclination angle. If the inclination angle of the swash plate exceeds the predetermined maximum inclination angle, the piston may collide against a plate that forms a suction valve.
Even if the refrigerant gas is sufficient in the variable displacement compressor, the operation of the variable displacement compressor at high rotation speed and at high displacement is undesirable in view of reliable operation In the variable displacement compressor including the hinge mechanism as disclosed in Unexamined Japanese Patent Publication No 2004-108245. the inclination angle of the swash plate may exceed the predetermined maximum inclination angle because of a great inertial force of the piston.
The present invention is directed to avoidance of large displacement operation of a variable displacement compressor with insufficient refrigerant gas and at high rotation speed.